1. Field of the Invention
The present invention relates to an optical recording medium having an area where grooves are formed along a recording track and an area where a signal normalized at a predetermined bit interval T is recorded as a pit train. The present invention relates also to a stamper for use to produce the optical recording medium and a method of producing the stamper.
2. Description of the Related Art
As such an optical recording medium, there are available optical discs to and/or from which a signal is optically written and/or read. Such optical discs include a read-only optical disc having preformed in a disc substrate thereof a pit train corresponding to a recording signal, such as a compact disc, laser disc, etc. The mark length of each of pits in the pit train is a length of a to-be-written signal normalized at a predetermined bit interval T. That is, the mark length of each pit in the pit train is an integral multiple of the bit interval T.
Also, the optical discs include a magneto-optical disc to and/or from which a signal is written and/or read under the magneto-optical effect, such as a mini disc, and a phase-change optical disc to and/or from which a signal is written and/or read using the phase change of a recording layer thereof, such as DVD. The optical discs to which a signal can be written, such as the magneto-optical disc and phase-change optical disc, have normally formed in a disc substrate thereof grooves extending along a recording track. The xe2x80x9cgroovexe2x80x9d referred to herein is a so-called guide recess formed along a recording track to enable mainly a tracking servo.
For the tracking servo of the optical disc having grooves formed therein, the push-pull method is applied for example. In the push-pull method, the tracking servo is effected based on a push-pull signal derived from a reflected and diffracted light from the groove. The push-pull signal is provided by detecting a reflected and diffracted light from the groove by means of two photodetectors disposed symmetrically with respect to the track center and calculating a difference between outputs of the two photodetectors.
The process of producing the aforementioned optical disc consists mainly of a step at which a stamper is prepared which is intended for use to produce the optical disc and has a surface shape corresponding to a pit-land pattern including pit trains and grooves which are to be formed in the optical disc (this step will be referred to as xe2x80x9cmastering stepxe2x80x9d hereinafter), a step at which the surface shape of the stamper is replicated to a disc substrate, and a step at which a recording layer, protective layer, etc. are formed on the disc substrate having replicated thereto the surface shape of the stamper.
Normally at the mastering step, a disc-shaped glass substrate having a polished surface is cleaned and dried, and then it is coated with a photoresist. Next, the photoresist is exposed to a laser light to form a latent image corresponding to a pit-land pattern including pit trains and grooves which are to be formed in an intended optical disc. It should be noted that the formation of a latent image by exposure of a photoresist to a laser light is generally called xe2x80x9claser cuttingxe2x80x9d and an apparatus to form a latent image by exposure of a photoresist to a laser light is generally called xe2x80x9claser cutting equipmentxe2x80x9d.
Thereafter, the photoresist in which the latent image has been formed by the exposure to the laser light is developed to form, on the glass substrate, the pit-land pattern including the pit trains and grooves. A metal layer is formed on the pit-land pattern by electrocasting, and thereafter, the metal layer is separated. The metal layer having replicated thereto the pit-land pattern formed on the glass substrate is thus provided as a master for use to produce an optical recording medium. Note that the master thus produced is generally called xe2x80x9cstamperxe2x80x9d.
There are various requirements for the optical discs. One of such requirements is to provide an optical disc having both a writable area and a read-only area formed therein. To meet this requirement, there has been proposed an optical disc having an area where grooves are formed along a recording track (will be referred to as xe2x80x9cgroove areaxe2x80x9d hereinafter) and an area where a recording signal is recorded as a pit train (will be referred to as xe2x80x9cpit train areaxe2x80x9d hereinafter). In this optical disc, the groove area is the writable area while the pit train area is the read-only area.
However, if in such an optical disc, the width of each of pits forming together the pit train formed in the pit train area (will be referred to as xe2x80x9cpit widthxe2x80x9d hereinafter) is equal to that of each of grooves formed in the groove area (will be referred to as xe2x80x9cgroove widthxe2x80x9d hereinafter), the amount of a push-pull signal from the pit train area will be nearly a half of that of a push-pull signal from the groove area. Therefore, no stable tracking servo is possible in the pit train area.
On the other hand, if the pit width is increased, the amount of a push-pull signal from the pit train area will be larger. Therefore, when the pit width is sufficiently wider than the groove width, the amount of push-pull signal from the pit train area can be nearly equal to that from the groove area, whereby a stable tracking servo is possible in the pit train area as well.
If the pit width is increased, however, when a signal recorded as a pit train in the pit train area is read, the asymmetry of the read signal will be lower. For example, if a recording signal is 1-7 modulated and the 1-7 modulated signal is recorded as a pit train while the pit width is increased to about a half of the track pitch so that the push-pull signal amount from the pit train area will be approximate to that from the groove area, the asymmetry of a signal read from the pit train will be less than xe2x88x9215%. In this case, it is difficult to read a signal stably.
In such a conventional optical disc having formed therein both the groove area as a writable area and the pit train area as a read-only area, the equalization of the push-pull signal amount from the groove area to that from the pit train area is not compatible with the improvement in asymmetry of a read signal from the pit train area. Note that the equalization of the push-pull signal amount to that from the groove area will be referred to as xe2x80x9cequation of push-pull signal amountxe2x80x9d in the description made herebelow.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing an optical recording medium having formed therein both a groove area and a pit train area and in which the equation of push-pull signal amount is compatible with the improvement in asymmetry of a read signal from the put train area.
It is another object of the present invention to provide a stamper for use to produce such an optical recording medium.
It is further object of the present invention to provide a method of producing such a stamper.
According to the present invention, there is provided an optical recording medium having formed therein an area in which grooves are formed along a recording track and an area in which a signal normalized at a predetermined bit interval T is recorded as a pit train,
the mark length of each of pits forming together the pit train being shorter than that of each of pits for a to-be-written signal normalized at the predetermined bit interval T; and
the mark length of a shortest one of the pits forming together the pit train being 64.8 to 80.0% of the shortest mark length for the to-be-written signal normalized at the predetermined bit interval T.
In the above optical recording medium, the mark length of each of pits forming together the pit train is shorter than that of each of pits for a to-be-written signal normalized at the predetermined bit interval T and the mark length of a shortest one of the pits forming together the pit train is 64.8 to 80.0% of the shortest mark length for the to-be-written signal normalized at the predetermined bit interval T. By thus defining the mark length of each pit, the asymmetry of a read signal from the pit train area will not be lower even when the pit width is increased to equate the push-pull signal amount from the pit train area.
Note that on the assumption that the width of each of pits forming together the pit train is tp and the groove width is tg, they should preferably meet a relation 1.22 less than tp/tgxe2x89xa61.84 in the above optical recording medium. Satisfaction of this relation makes it possible to equate the push-pull signal amount to a sufficient level for a stable tracking servo.
Also according to the present invention, there is provided a stamper for use to produce an optical recording medium having formed therein an area in which grooves are formed along a recording track and an area in which a signal normalized at a predetermined bit interval T is recorded as a pit train, the stamper having a groove pattern being a pit-land pattern corresponding to the grooves and a pit train pattern being a pit-land pattern corresponding to the pit train,
the mark length of each of pit patterns forming together the pit train pattern being shorter than that of each of pits for a to-be-written signal normalized at the predetermined bit interval T; and
the mark length of a shortest one of the pit patterns forming together the pit train pattern being 64.8 to 80.0% of the shortest mark length for the to-be-written signal normalized at the predetermined bit interval T.
Note that on the assumption that the width of each of pit patterns forming together the pit train pattern is tp and the groove pattern width is tg, they should preferably meet a relation 1.22 less than tp/tgxe2x89xa61.84 in the above stamper.
Also according to the present invention, there is provided a method of producing a stamper for use to produce an optical recording medium having formed therein an area in which grooves are formed along a recording track and an area in which a signal normalized at a predetermined bit interval T is recorded as a pit train, the stamper having a groove pattern being a pit-land pattern corresponding to the grooves and a pit train pattern being a pit-land pattern corresponding to the pit train, the method including the steps of:
making the mark length of each of pit patterns forming together the pit train pattern shorter than that of each of pits for a to-be-written signal normalized at the predetermined bit interval T; and
making the mark length of a shortest one of the pit patterns forming together the pit train pattern 64.8 to 80.0% of the shortest mark length for the to-be-written signal normalized at the predetermined bit interval T.
Note that on the assumption that the width of each of pit patterns forming together the pit train pattern is tp and the groove pattern width is tg, they should preferably meet a relation 1.22 less than tp/tgxe2x89xa61.84 in the above method.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.